1. Field of the Invention
The subject invention generally pertains to a derrick of a hoist and more specifically to an apparatus for sensing a load applied to the derrick.
2. Description of Related Art
Hoist and derrick systems used for drilling or servicing wells often handle loads ranging from about a hundred pounds for lighter well components, such as sucker rods, to a hundred tons or more for a string of well casings.
The load on the derrick is usually monitored in some way to avoid applying excessive lifting force that may damage well components and to avoid overloading the hoist and derrick.
The accuracy of the load measurement is preferably sufficient to differentiate 200 pounds of load. Such accuracy, however, can be difficult to achieve for a common derrick having a rated hook load of 200,000 pounds and having a designed ultimate strength of over 400,000 pounds, considering a safety factor of at least two. 200 pounds is a mere 0.05% of a 400,000-pound derrick, so loads varying by 200 pounds may be difficult to differentiate using conventional means. Consequently, a need exists for an improved, more accurate device for sensing the load on a derrick.